The present invention relates to a method for decontaminating a liquid by evaporation, particularly a liquid containing radioactive substances, where the evaporated portion of the liquid is conducted through a plurality of washing liquid baths and the washing liquid flows through the individual baths substantially in countercurrent to the evaporated portion (vapor) of the liquid.
Due to the requirements for environmental protection, it has become increasingly necessary to remove contaminants of liquids containing radioactive substances before they are released into public waters. Such purification of the liquid becomes necessary in particular where liquids or waste waters, laden with radioactive substances, are involved. Radioactivity is measured in micro-Curies per cm.sup.3 (.mu.Ci/cm.sup.3) or in Curies per m.sup.3 (Ci/m.sup.3). According to generally applicable standards, waste waters containing unknown and/or radioactive substances with a long half-life which are discharged into public waters should have a radioactivity below 10.sup.-7 Ci/m.sup.3.
It is known that in radioactive waste waters it is not the water which carries the radioactivity; rather such radioactivity is carried by dissolved salts or suspended solids, such as, for example, undissolved salts, oxides, abraded metal particles, etc.
In a known process, the radioactive waste water is evaporated and the vapor, together with drops of the liquid it carries along, is passed through a washing liquid. Thus, a majority of the drops are transferred to the washing liquid. The drops are mixed with the washing liquid so that the radioactive substances contained in the drops are diluted in the washing liquid, reducing the radioactivity per unit volume, for example per cm.sup.3. Moreover, the drops carried along by the vapor are separated in drop separators arranged above each plate of the separator. These drop separators are designed as demisters and are manufactured of pressed mats of steel wool or similar materials. By separating the drops in demisters, the latter are enriched with contaminants laden with radioactive substances which adhere to the fabric of the demisters. This increases the radioactivity within the column. Experience has shown that simply rinsing these columns, i.e. the plates and the demisters, does not result in sufficient removal of these radioactive substances.
Columns are also known which employ a plurality of plates on which there is arranged, in addition to an inlet and outlet weir permitting a uniform distribution of the liquid on the column plates, a baffle plate which is fastened above the plate of the column. This baffle plate permits deflection of the vapors passing through the plates or through the liquid collected on the plates whereby liquid drops carried along by the stream of vapor are in part separated from the vapor stream. With this type of drop separation it is accomplished that no impermissible enrichment of radioactive substances can take place at any point in the column.
In using the above-outlined two known methods, it has been found, however, that drop separation alone cannot result in the required final concentration in the distillate; this could be achieved only by significantly increasing the quantity of washing liquid. Yet, this measure requires significantly greater amounts of heat energy, since the additional washing liquid must be evaporated together with the contaminated liquid. Furthermore, it is necessary to enlarge the plate diameter through which the vapor and the drops carried along therewith must pass. This measure likewise makes the entire system more costly.